Process for treating polluted liquid



y 8, 1954 A. A. KALINSKE ROCESS FOR TREATING POLLUTED LIQUID Filed Dec. 15, 1950 3. n MdDUH N MGDE INVENTOR. anfow/inske AGENT Patented May 18, 1954 PROCESS FOR TREATING POLLUTED LIQUID Anton A. Kalinske, Tucson, Ariz., assignor to Infilco Incorporated, Tucson, Ariz., a corporation of Delaware Original application April 2, 1949, Serial No. 85,102. Divided and this application December 15, 1950, Serial No. 201,036

Claims. 1

This invention relates to a process for the treatment of polluted liquids, such as sewage and other Waste liquids and polluted surface waters, and is a division of my copending application, Serial No. 85,102, filed April 2, 1949. More specifically, the invention relates to a novel method for the biological purification of such liquids by what is generally called activated sludge, and the clarification of the purified liquid.

For purposes of simplification and illustration I will refer herein sometimes to sewage; it should however be understood, that the term sewage is used herein in broadest sense, to denote polluted liquids irrespective of whether they are treated to make their discharge to waste or their use permissible. Thus, the term sewage, as used herein, denotes in addition to domestic sewage, any kind of liquid wastes, mixtures of such wastes or of sewage with such waste or wastes, as well as polluted surface Waters.

When using herein terms such as holding time or treating time I wish to denote detention based on throughput only.

It is an object of this invention to provide an improved activated sludge process.

A specific object of the invention is to reduce the treating time of an activated sludge process and thereby reduce the size and cost of the apparatus and of the site for its installation, and the operating cost of the plant.

Another object of the invention is to provide better utilization of the compressedair in an activated sludge plant whereby the quantity of compressed air used can be much smaller than the quantity heretofore used in conventional diffused air plants.

Another object is to provide an activated sludge process wherein raw or settled waste liquid and air or oxygen under pressure are instantaneously dispersed throughout the entire contents of the aeration chamber.

Another object is to provide an activated sludge process wherein the clarification of the mixed liquor is obtained by separation of clarified liquid from a circulating body of mixed liquor.

Another object of the invention is to provide an activated sludge process wherein the activated sludge is held in suspension as long as it is used in the process, and only waste sludge leaving the process is allowed to settle.

Another object of the invention is to provide a process wherein the raw or settled waste liquid to be treated and the oxygen used in its treatment are introduced into the aeration chamber ccnjointly or at adjacent points and are quickly dispersed through a volume of mixed liquor under treatment several times the volume of newly added waste liquid.

In its broadest sense. the activated sludge process may be said to involve treating raw or pre-clarified waste liquid, which may contain suspended, colloidal and dissolved pollute, with a sludge containing aerobic bacteria in the presence of sufiicient oxygen to keep the bacteria in an active condition, and with sufficient agitation to maintain the sludge and waste liquid thoroughly mixed. Many diverse theories have been stated regarding the exact functioning of the activated sludge process, however it is now generally recognized that the process involves both bacterial oxidation of organic matter in suspension and solution, and agglomeration of suspended and colloidal solids.

In actual practice of the conventional activated sludge process the polluted liquid is usually settled in a primary clarifier to permit the depositing and removal of the more readily settleable solids. The overflow from the primary clarifier flows to an aeration basin or basins where it is mixed with activated sludge separated from previously aerated polluted liquid, and aerated, usually for periods of about eight to ten hours. The aerated mixture of activated sludge and polluted liquid, the so-called mixed liquor, is then commonly settled in a secondary clarifier for about two hours, and the eiiluent of the secondary clarifier is usually suitable for discharge into a water course. A portion of the sludge which settles in the secondary clarifier is returned to the aeration basin and provides the activated sludge used in the treatment of the polluted liquid therein, While another portion, which is in excess of the sludge needed in the aeration basin, is wasted.

The aeration of the polluted liquids has been carried out mainly by two different methods. In one method compressed air is diffused into the mixed liquor on its way through the aeration basin from the inlet end to the outlet end, usually through porous plates or nozzles arranged along the length of the aeration basin. In the other method mechanical equipment is used for introducing oxygen from the atmosphere into the mixed liquor and at the same time provide the necessary agitation for mixing of sewage and activated sludge and for keeping or maintaining the sludge in suspension. This is done in various ways, as by agitation of the liquid surface in the aeration basin by heaters, paddles and the like in such a manner as to induce circula- 3 tion of the tank contents, or by spraying of a portion of the mixed liquor over the liquid surface, or by drawing surface liquid into the mixed liquor in the aeration basin. It has also been proposed to combine diffusion of compressed air with paddle agitation of the liquid surface.

In the diffused air method of aeration the oxygen is generally made available in a progressive manner, that is the mixed liquor on its way through an elongated aeration basin is dosed with successive portions of air, and, vice versa, each portion of oxygen supplied is diffused only in a small portion of the contents of the aeration basin. The same is true of paddle aeration, where the sewage usualy flows through long channels and is acted upon by successive paddles. In the so-called mechanical aerators, air from the atmosphere is drawn into the mixed liquor and mixed with the entire contents of the aeration basin. However, the quantity of oxygen provided in this manner is relatively small and therefore a very long holding time is needed. Further, while the mechanical aerators do not in and of themselves use progressive aeration as do diffused air aerators, they are, except in very small plants, used in series, and then have the same characteristic of progressive aeration by sucessively introducing small quantities of air into the mixed liquor, each quantity of air being introduced only into a portion of the entire mass of mixed liquor under treatment.

In my new process aeration is carried out by means of air, or oxygen, under pressure. However, instead of introducing the air, or oxygen, progressively, as in the conventional diffused air methods, the air, or oxygen, is introduced in one location of the aeration chamber where it is immediately picked up by, and dispersed through the circulating contents of the chamber. The sewage to be treated is introduced in like manner, so that there is obtained an almost instantaneous mixing and dispersion of entering sewage and oxygen with each other and with the entire contents of the aeration basin. Due to this method of operation each volume of newly entering sewage is contacted with and acted upon by much larger quantities of air and activated sludge than are introduced into, or held in, the process per volume of sewage to be treated.

Prior art activated sludge treating processes have in common that the final separation of the mixed liquor into purified efiluent and activated sludge (part of which must be returned to the aeration basin) is effected by depositing the sludge under quiescent conditions, be it in a settler or on a filter.

The use of a fully deposited sludge as the source of activated sludge for the treatment of sewage is objectionable in various respects. In the first place, the sludge has to be resuspended and this involves loss of power as compared with holding the sludge in suspension. Further, settling impairs the faculty of the sludge of assisting in the agglomeration of fine particles into larger sludge flocs. t is a well known fact that once solids have been fully deposited as a sludge, whether they be thereafter resuspended or not, their capacity of assisting in coagulation and in agglomeration is not nearly as high as that of unsedimented suspended solids. Further, and most important, such separation of sludge by depositing necessitates holding the sludge for prolonged periods under non-aerobic conditions, whereby the sludge becomes somewhat dormant and the bacteria are not in the most vigorous state for active biological oxidation of the sewage pollute. Such deposited activated sludge must either be aerated before its reuse in the aeration basin, to re-activate the bacteria, or the oxygen provided in the inlet portion of the aeration basin will be consumed to the greatest part in activating the bacteria and the biological purification of the sewage will proceed relatively slowly.

In my process this drawback is avoided. The activated sludge formed in the treatment of polluted liquid in the aeration chamber and used therein over and over again, is not permitted to deposit, or even remain quiescent, or exposed to anaerobic conditions at any time, but is maintained in suspension and circulation under aerobic conditions in the treating zone until it is discarded as waste sludge. According to my invention, only the excess sludge is permitted to deposit in a quiescent concentration chamber from where it is sent to waste. Since only a small quantity of sludge is allowed to settle, the concentrator can be located within the treating apparatus and the cost and space for separate final clarifiers and for pumping return sludge can be saved.

Briefly, my invention comprises the rapid dispersion of oxygen and sewage throughout a large body of mixed liquor in an aeration zone, the continuous circulation of mixed liquor from the outlet portion of the aeration zone to its inlet portion, which circulation is at least partially effected by air lift, the separation of undissolved gas from the circulating liquid, the separation of clarified sewage from the circulating mixed liquor, the displacement of an output portion of clarified sewage by newly entering sewage and the withdrawal of waste sludge from the process.

I contemplate using compressed air, or pure oxygen under pressure. Therefore, it should be understood that where I speak of air herein, this term is used to denote also pure oxygen, and vice versa, where I speak of oxygen, this term denotes both the oxygen contained in compressed air and pure oxygen introduced per se.

The entire holding time for the combined bi ological purification and clarification steps for treatment of normal strength domestic sewage by my process may be of the order of about two and one-half hours, with results which compare favorably with those obtained with holding times of from eight to ten hours for the biological purification and about two hours for final clarification in the conventional activated sludge process and. with holding times of about three or more hours for the biological purification and about two hours for clarification in the high-rate activated sludge process.

My invention will be more fully understood by reference to the drawings which form a part of this specification and wherein Figure 1 is a cross sectional view of a preferred embodiment of an apparatus suitable for carrying out my invention;

Figure 2 is an enlarged cross sectional view of a modification of the adjustable sleeve of Figure 1; and

Figure 3 is a partial diagrammatic plan view of an elongated tank suitable for carrying out my invention.

The preferred apparatus for carrying out the process of my invention comprises a tank, which may be of any suitable size and shape but in Figure 1 is shown for purposes of illustration as a round tank I having a side wall II and a substantially flat bottom I2. The lower portion of the tank wall may be sloped inwardly as shown at I3 to facilitate flow to the center of the tank and avoid deposits on the floor along the wall H. A valved drain [4 may lead from the tank to permit draining of the apparatus for inspec tion, repair and the like. A peripheral launder l5 communicates with an efiluent pipe is and establishes the liquid level Lin the tank l0.

Axially aligned within the tank H] is a vertically extending partition structure .29 comprising a frusto-conical hood 2! spaced above the floor I2 and a cylindrical wall or inner tube 22 ex tending upwardly from the upper portion of the hood 2| as shown. An annular wall or outer tube 23 surrounds the inner tube 22 and extends from an elevation above the liquid level downwardly to an elevation adjacent the lower end of the inner cylindrical wall 22. The partition structure 29 may be supported in spaced relationship to the tank bottom by any suitable means, such as legs, or brackets 24 as shown. The outer tube 23 may derive support from the inner tube, as by brackets 25, which may also serve as bailies, or it may be supported from a beam, or bridge 25 spanning the tank it.

The partition structure 26 and the outer tube 23 form in the tank It an aeration chamber 30 located under the hood 2| which is open at its bottom to the lower portion of the tank, and a passageway 3| for mixed liquor leading from. the upper portion of the aeration chamber 3i] to the liquid surface and thence back to the lower portion of the tank. The passageway 3| comprises an upflow section 32 within the inner tube 22 and a downfiow section 33 within the outer tube 23. The downflow section 33 must be of suflicient width that the downward flow of the mixed liquor returning to the lower portion of the tank is relatively slow in order to permit undissolved gas entrained in the mixed liquor to escape to the surface countercurrently to the flow of the liquor.

It is desirable to control the rate of flow through the passageway 3!. by adjusting the elevation at which flow from the inner tube 22 to the outer tube 23 takes place. As shown in Figure l a sleeve 35 is provided Which fits snugly into the inner tube 22. The

sleeve may be supported from the bridge 26 by turnbuckles, or the like, so that it can be raised or lowered as desired. The sleeve 35, in effect,

forms a vertically adjustable weir 36.

Instead of using the upper edge 35 of the sleeve 35 as an overflow weir, the sleeve may be aperindividual vertically movable gates may be pro vided for the several orifices. Obviously, the orifices 31 could be in the inner tube and the sleeve be made solid with the same result.

Radial bafiies 38 are provided under the hood 2! and extend inwardly therefrom, to reduce the rotation of liquid along the hood.

Axially aligned within the partition structure Ell is a shaft ill which may be journalec. in

This can be done driven by any suitable means, such as a motor- 6 reducer 42 which may be supported above the tank as on the beam or bridge 26. A rotor 43 is rigidly afiixed to, and rotatable with the shaft.

The rotor 43 may comprise a horizontal plate 44 and a plurality of vertically extending blades 55 mounted on the underside of the plate 44. The blades 45 .are spaced around the periphery of the plate M and extend approximately radially inwardly therefrom only partway to the shaft Ml. Preferably, the width of the blades may be approximately one-seventh of the diameter of the plate-and their length from about one-fourth to about one-third the diameter of the plate. Eight blades are indicated in the drawing for purposes of illustration, but the actual number of blades used will depend on the size of the rotor which again will depend on the size of the aeration chamber and the capacity of the apparatus. The peripheral distance between blades preferably should not exceed 15 to 20 inches, so that with large rotors a much larger number of blades is desirable.

Such a rotor provides a very efiicient means for breaking up the air into very fine bubbles and for rapidly dispersing the entering air and sewage through, and intimately mixing them with, the entire body of mixed liquor in the aeration chamber. The rotor 43 is so positioned in the aeration chamber 30 and so proportioned to the size of the aeration chamber that the flow set up by its rotation embraces all the liquid in the aeration chamber. The cyclic flow pattern set up by the rotor includes a radial outward discharge flow at the level of the blades and a radial inward suction flow along the bottom of the tank It.

A propeller it is shown affixed to the shaft 49 within the inner tube 22. Usually, such a propeller will not be needed, as the circulation through the tube 22, 23 can be maintained by air lift. However, in some cases, especially when pure oxygen is used in my apparatus for the aeration of the mixed liquor, there may not remain a suihcient amount in the form of undissolved gas bubbles to establish a lift for circulating the mixed liquor through the passageway 3|. In such case a propeller 46 may be used to supplementthe lift due to rising gas bubbles.

t is essential in my apparatus that the liquid to betreated and the air or oxygen used in the treatment be introduced into the suction flow to the rotor 43, so that both the liquid and the oxygen are immediately picked up and dispersed by the rotor throughout the entire contents of the aeration chamber. The inlets for air and liquid can discharge separately, at spaced points, but the oxygen should always be discharged below the plate is to prevent its escape before it has been dispersed through the mixed liquor. One way of doing this is to make the shaft it hollow and introduce the compressed air or oxygen thereth'rough directly into the center of the rotor, in known manner. Preferably, however, oxygen and sewage are introduced at adjacent points or, shown in Figure l, at a common point of discharge 553 just below the rotor. Obvi ously, but not necessarily, the air inlet line 51 can discharge into the sewage inlet conduit 52 at any suitable distance from the tank, whereby a gore-mixing of sewage and air and pre-aeration of the sewage is chested.

While so far I have described for purposes of illustration and exemplification the use of a single rotor, which is preferred in noneelongated' tanks, I contemplate using two or more rotors in longitudinal tanks. Such an arrangement is shown diagrammatically in Figure 3. The rotors 43a and 43b are positioned at the longitudinal axis of the tank Illa under the elongated hood tile. The incoming sewage and air are introduced in parallel into the suction fiow of the two rotors, as shown, each rotor receiving half of the entering sewage and air. Thus, also in this form of apparatus, the air and sewage are immediately dispersed through, and intimately mixed with, the entire contents of the aeration chamber in the same manner as described for a round tank.

A sludge concentrator -3 is provided in the lower portion of the tank Ill. A waste sludge pipe 54 provided with a valve 55 leads from the concentrator 53 to outside the tank It. The concentrator 53 can be quite small, as it serves only for thickening the excess or waste sludge which is withdrawn from the process, this being the only sludge permitted to settle in my apparatus. The balance of the activated sludge returns from the outer draft tube to the aeration chamber without settling. By regulating the quantity of activated sludge withdrawn through the waste sludge line 54 the mixed liquor surface can be maintained at any desired elevation. Preferably, the mixed liquor surface is maintained approximately at the elevation of the lower end of the outer draft tube, so that clarified treated sewage separates from a laterally flowing stream of mixed liquor. This separation is well defined, with a sharp demarcation line between the clarified sewage above and the turbid liquor below.

In carrying out the process in the apparatus described, the sewage to be treated may be raw sewage, but preferably is settled raw sewage, i. e., sewage from which settleable solids have been previously removed in conventional manner, as in a grit chamber or a primary clarifier, not shown. The lower portion of the tank, and the upfiow section 32 and downfiow section 33 of the passageway 3| are normally completely filled with mixed liquor, and the upper outer portion of the tank is normally filled with purified and clarified sewage. Rotation of the rotor -43 by the motor-reducer 42 sets up a cyclic flow of mixed liquor throughout the aeration chamber 30 and the underlying portion of the tank. The sewage arriving through conduit 52 and air or oxygen under pressure arriving through pipe 5| are discharged into the suction flow to the rotor 43 and quickly dispersed by the rotor through the entire body of mixed liquor in the aeration chamber 30. The air is broken up in minute bubbles from which oxygen is readily absorbed by the mixed liquor.

The gas bubbles rising through the upfiow section 32 produce an air lift action which causes a circulation of mixed liquor through the inner tube 22 and the outer tube 23 back to the lower part of the tank. Due to the air lift action the liquid level L in the inner tube is somewhat higher than the liquid level L established by the overflow of launder 15. If lift due to the rising gas is not suflicient to maintain the circulation, the propeller 45 may be used to supplement or replace it. The mixed liquor flows from the inner tube 22 into the outer tube 23 over the weir 36 formedby the adjustable sleeve 35, or through the submerged orifice or orifices 31 in the sleeve 3511. From the surface of the mixed liquor flowing laterally from the bottom of outer tube 23 an output portion of treated clarified sewage separates and passes upwardly into the clarified liquid zone from where it is withdrawn through 8 the launder I5 and effluent conduit l8. The balance of mixed liquor returns downwardly to the bottom portion of the tank and is picked up by the suction fiow to the rotor 43 and drawn back into the aeration chamber to be redispersed therethrough.

During the flow through the inner and outer tubes undissolved air separates in form of bubbles from the mixed liquor and escapes through the open top of the tubes, carrying with it carbon dioxide formed by the bacterial activity in the sludge. Most of the air will escape during the upfiow. However, the downfiow must be sufficiently slow to permit the counter-current escape of air bubbles still entrained in the mixed liquor in order to prevent their release at the mixed liquor surface where they would disturb proper clarification. This counter-current flow of mixed liquor and air offers additional time for oxygen adsorption by the mixed liquor.

The mixed liquor surface from which the clarified sewage emerges may be maintained at the desired elevation by regulating the quantity of sludge wasted through concentrator 53.

The rate of circulation through passageway 3! is controlled by adjusting the height of the weir 36 or the size of the orifices 31 through raising or lowering of the sleeve 35 or 35a, respectively. Proper control of the circulation rate is quite essential in this apparatus. If the circulation rate is excessive too much disturbance at the mixed liquor surface will result in consequent poor clarification. If, on the other hand, the circulation is insufficient, there will be a tendency for the intense agitation under the hood to be carried into the outer portion of the tank and disturb proper clarification.

Pilot plant tests of my process carried out in my new apparatus, shown herein, have been conducted for several months continuous operation, treating screened domestic sewage taken from a conventional sewage plant. The B. O. D. of the screened raw sewage varied from about to 200 p. p. m. (parts per million). The pilot plant produced an effluent having a B. O. D. of 10 to 25 p. p. m. with a total retention time of two and one-half hours. The B. O. D. and suspended solids reduction obtained were consistently practically identical with those obtained in the conventional plant treating the same sewage with mechanical aerators in series but having a retention time of about ten to twelve hours. The sludge produced was denser than the sludge from the conventional mechanical aeration plant and its Mohlmans Index was lower. The air requirement of my apparatus and process is very low. My pilot plant results were obtained using one-half cubic foot of air per gallon of sewage treated, while in a conventional diffused air plant one cubic foot per gallon is needed to produce the same results.

These results show an enormously increased rate of oxidation and clarification of sewage treated by my process and apparatus and a very favorable rate of oxygen utilization. The rapid oxidation obtained is due in part to the rapid dispersion of air and sewage to be treated through, and intimate mixture with, the entire body of mixed liquor in the aeration zone, whereby each volume of sewage is contacted wih many times the quantity of air and activated sludge that is introduced per volume of sewage. In conventional activated sludge plants each volume of sewage comes in contact only with the volumes of air and activated sludge introduced for treatment of such volume of sewage.

Another important factor contributing to the rapid oxidation of the sludge is the complete elimination of any quiescent holding period, or stagnant condition, between periods of aeration of the sludge in the aeration chamber. Due to the continuous circulation of mixed liquor described, the bacterial population of the sludge is constantly held under aerobic conditions and the organisms are enabled to work at their maximum efficiency.

The rapid clarification obtained in my apparatus is due to the fact that clarification is ob- .tained by separation of clarified sewage from a laterally flowing stream of mixed liquor. The density and good settling characteristics of the sludge produced further enhance the clarification rate.

Due to these high oxidation and clarification rates, relatively small apparatus can handle large volumes of sewage. In large plants several of my apparatus may be used and will usually be operated in parallel. However, they may be connected in series, if desired. When treating, for example, waste waters which are difficult to purify, it may be advantageous to use a stage treatment. Furthermore, my process can be used for treatment of polluted liquids that have had a prior treatment by other methods, such as chemically, or on a trickling filter. Therefore, while usually raw or raw-settled sewage will be treated in my apparatus, I wish it to be understood that'when I refer tothe liquid to be treated I include in this term also polluted liquids that have been partially treated whether by the same or by different methods.

It will be seen that I have invented a new process for the treatment of polluted liquids with activated sludge whose results are comparable to thoseof conventional plants but are obtained in a fraction of the time needed in conventional plants, and with a smaller amount of air per gallon of sewage treated.

Many modifications and variations of the invention, as set forth herein, may be made without departing from the spirit and scope of the invention. Accordingly, the specification herein is to be considered for purposes of illustration rather than of limitation.

I claim:

1. An activated sludge sewage treating process comprising continuously maintaining an aeration zone filled with a mixture of sewage undergoing treatment and activated sludge formed in the process, continuously applying mechanical energy to said mixture to circulate the mixture in said aeration zone in predominantly horizontal planes including a radially outward flow from the center of said aeration zone and a radially inward flow to the center at a lower level of said aeration zone, continuously delivering sewage to be treated into said inward flow, and simultaneously delivering oxygen under pressure to the entering sewage in sufficient quantity for its treatment, thereby dispersing said sewage to be treated and said oxygen through the entire contents of said aeration zone, maintaining a controlled substantially vertical circulation of a portion of said mixture in excess of the throughput through a predominantly vertical path leading out of the aeration zone from an upper level thereof and returning to a lower level of said aeration zone, said circulation being at least partially efi'ected by air lift, separating undissolved gas from the circulating mixture in one part of said path and clarified sewage in another part of said path, returning the balance or said circulating mixture including activated sludge left behind by the clarified sewage to said aeration zone, thereby building up the activated sludge concentration in said aeration zone, and separating excess activated sludge from said mixture and withdrawing it to waste.

2. An activated sludge sewage treating process comprising maintaining a submerged aeration zone filled with a large body of mixed liquor comprised of sewage undergoing treatment and activated sludge formed in the process, applying mechanical energy to said body of mixed liquor to establish in said aeration zone a closed cycle fiow in predominantly horizontal planes having radial outward and radial inward components and embracing said entire body of mixed liquor, introducing into, and immediately dispersing through, said closed cycle flow newly entering polluted liquid to be treated and air under pressure, circulating a portion of said mixed liquor by air lift action through a predominantly vertical path leading from the aeration zone upwardly to adjacent the sewage surface and back to the aeration zone, separating gases from said circulation in one part of said path, separating purified and clarified sewage from said circulating mixed liquor in another part of said path and withdrawing the clarified sewage from the process, returning the balance of said circulating mixed liqucr including a portion of the activated sludge left behind by the clarified sewage to the aeration zone, and withdrawing excess activated sludge from the process to waste.

3. In the treatment of sewage by the activated sludge process wherein sewage is aerated and mixed with activated sludge derived from previously treated sewage in an aeration zone and the resulting mixed liquor is clarified, the improvement which comprises maintaining in a submerged aeration zone a closed cycle fiow of mixed liquor comprising sewage undergoing treatment and activated sludge formed in the treatment in predominantly horizontal planes having radial outward and radial inward flow components and embracing the entire contents of said aeration zone, discharging the sewage to be treated and the air used in the process into said radial inward flow and thereby dispersing them through the entire contents of said aeration zone, simultaneously maintaining a circulation of mixed liquor in predominantly vertical planes out of said aeration zone and back into it, the volume of said circulation being in excess of the volume of the throughfiow, separating an output portion of clarified treated sewage from said circulating mixed liquor while retaining the activated sludge separated from said output portion of clarified sewage in said circulating mixed liquor, whereby said activated sludge is returned to the aeration zone and the sludge concentration therein is built up, maintaining throughout said circulation a velocity sufficient to prevent sedimentation of solids from the circulating mixed liquor but insufficient to disturb said separation of clarified sewage, and controlling said concentration by withdrawing excess activated sludge to waste.

4. A process for the biological purification oi polluted liquids such as sewage wherein sewage to be treated is mixed and agitated with oxygen containing gas and activated sludge derived from previously treated sewage for a period of time, the mixture is then separated into purified effluent and activated sludge, a portion of which sludge is reused in the treatment of a further volume of sewage, characterized by maintaining a suspension of sewage undergoing treatment and activated sludge formed in the treatment in an aeration zone in the lower portion of a body of sewage, applying mechanical energy to said suspension to prevent sedimentation of solids and to establish a fiow pattern in predominantly horizontal planes, having radial outward and radial inward flow components and embracing the contents of said aeration zone, introducing newly entering sewage to be treated and oxygen containing gas under pressure into said radial inward fiow whereby they are immediately dispersed through, and incorporated in, said suspension, circulating a portion of said suspension in excess of the volume of incoming sewage through a predominantly vertical path leading from said aeration zone to the top of said body of sewage and back to said aeration zone with a velocity sufiicient to prevent sedimentation of solids from the circulating suspension but insufiicient to disturb the separation of clarified sewage from the circulating suspension, separating clarified sewage from the suspension circulating through said path while retaining the activated sludge left behind by the clarified sewage in said circulating suspension returning to said aeration zone, thereby building up the concentration of activated sludge in said suspension, and controlling said concentration by withdrawing excess activated sludge to waste.

5. In the treatment of sewage by the activated sludge process wherein entering sewage to be treated is mixed and agitated with air and activated sludge derived from previously treated sewage for a period of time in an aeration zone,

and the mixture is then separated into purified efliuent and activated sludge, a portion of which sludge is reused for treatment of a further volume of sewage, the improvement which comprises maintaining an aeration zone filled with a suspension comprising a mixture of sewage undergoing treatment and activated sludge formed in the process, applying mechanical energy to the contents of the aeration zone to establish a circulation of said contents within said aeration zone in predominantly horizontal planes and including a radial outward flow and a radial inward flow, and to maintain said contents as a suspension, dispersing the entering sewage to be treated and air under pressure for the treatment through said circulating suspension, withdrawing a portion of said suspension in excess of the throughput upwardly from said aeration zone, and downwardly returning it thereto, separating a throughput portion of purified and clarified sewage from said withdrawn portion, the balance of said withdrawn suspension returning to said aeration zone including the solids left behind by said clarified sewage in unsedimented condition, whereby the activated sludge solids concentration of said suspension is built up, and controlling the solids concentration of said suspension in said aeration zone by withdrawing solids from said suspension to waste.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,348,125 Green May 2, 1944 2,348,126 Green May 2, 1944 2,364,023 Green Nov. 28, 1944 2,400,598 Prager May 21, 1946 2,429,315 Green Oct. 21, 1947 

1. AN ACTIVATED SLUDGE SEWAGE TREATING PROCESS COMPRISING CONTINUOUSLY MAINTAINING AN AERATION ZONE FILLED WITH A MIXTURE OF SEWAGE UNDERGOING TREATMENT AND ACTIVATED SLUGE FORMED IN THE PROCESS, CONTINUOUSLY APPLYING MECHANICAL ENERGY TO SAID MIXTURE OT CIRCULATE THE MIXTURE IN SAID AERATION ZONE N PREDOMINANTLY HORIZONTAL PLANES INCLUDING A RADIALLY OUTWARD FLOW FROM THE CENTER OF SAID AERATION ZONE AND A RADIALLY INWARD FLOW TO THE CENTER AT A LOWER LEVEL OF SAID AERATION ZONE, CONTINUOUSLY DELIVERING SEWAGE TO BE TREATED INTO SAID INWARD FLOW, AND SIMULTANEOUSLY DELIVERING OXYGEN UNDER PRESSURE TO THE ENTERING SEWAGE IN SUFFICIENT QUANTITY FOR ITS TREATMENT, THEREBY DISPERSING SAID SEWAGE TO BE TREATED AND SAID OXYGEN THROUGH THE ENTIRE CONTENTS OF SAID AERATION ZONE, MAINTAINING A CONTROLLED SUBSTANTIALLY VERTICAL CIRCULATION OF A PORTION OF SAID MIXTURE IN EXCESS OF THE THROUGHPUT THROUGH A PREDOMINANTLY VERTICAL PATH LEADING OUT OF THE AERATION ZONE FROM AN UPPER LEVEL THEREOF AND RETURNING TO A LOWER LEVEL OF SAID AERATION ZONE, SAID CIRCULATION BEING AT LEAST PARTIALLY EFFECTED BY AIR LIFT, SEPARATING UNDISSOLVED GAS FROM THE CIRCULATING MIXTURE IN ONE PART OF SAID PATH AND CLARIFIED SEWAGE IN ANOTHER PART OF SAID PATH, RETURNING THE BALANCE OF SAID CIRCULATING MIXTURE INCLUDING ACTIVATED SLUDGE LEFT BEHIND BY THE CLARIFIED SEWAGE TO SAID AERATION ZONE, THEREBY BUILDING UP THE ACTIVATED SLUDGE CONCENTRATION IN SAID AERATION ZONE, AND SEPARATING EXCESS ACTIVATED SLUDGE FROM SAID MIXTURE AND WITHDRAWING IT TO WASTE. 